As magnetic recording media on a rigid disk, there have been used .gamma.-hematite, Co-Pt, Co-Ni-Pt, and Co-Ni-P. The hematite media are coated with a resin on an aluminum or aluminum alloy substrate in a thin layer. The hematite media have been used for a long time in a lot of applications. But recently, strong demand for high recording density disk has occurred but the coated hematite media can not clear the demand. Instead of the coated hematite media, thin film media have been proposed for high recording density. The thin film media include plated media and sputtered media. The plated media, e.g. Co-Ni-P, have a disadvantage such as corrosion due to residual chemicals. Also, the corrosion causes an error of reading written data so that a very high density of recording cannot be accomplished. Consequently, plated thin film media for longitudinal recording have inherently high defect densities and poor corrosion resistance.
By contrast, sputtered thin film media have been expected to have a superior quality in every respect including: electrical characteristics, flyability, durability, wear, adhesion, defect density, corrosion resistance and reproducibility. As the sputtered thin film media, cobalt base alloys, e.g. Co-Ni-Pt, Co-Ni, have been proposed to be adapted for magnetic recording media for high longitudinal recording density. The Co-Ni-Pt is rather expensive because it contains platinum of about 10 at % which is an expensive element.
On the other hand, directly sputtered Co-Ni film tends to have its C-axes perpendicular to the sputtered plane so it is not appropriate to longitudinal recording.
For longitudinal recording media, magnetic recording media of sputtered thin film were proposed in some articles. According to Dr. H. Maeda, J. Appl. Phys. 53 (5) May 1982 p. 3735 "High coercivity Co and Co-Ni alloy films" and J. Appl. Phys. 53 (10) October 1982 p. 6941 "Effect of nitrogen on the high coercivity and microstructures of Co-Ni alloy films" and his patent application, Japanese laid-open patent 57-72307, cobalt metal or Co-Ni-alloy is sputtered in an atmosphere including nitrogen to form a thin film including nitrogen and then the thin film is heat-treated in vacuum or inert gas atmosphere to make it thin film magnetic recording media with good magnetic properties. Those papers teach that the sputtering process is required to be done on a substrate which is cooled by liquid nitrogen to obtain a sufficient nitrogen content in the sputtered thin film.
Like Dr. Maeda's experiment, when a glass plate or a quartz plate is used as the substrate, it is no problem to cool the substrate to liquid nitrogen temperature. The sputtering surface of the substrate is cooled to the temperature to make it easy to capture nitrogen in the sputtered film even though ion bombarding by sputtering causes to increase the temperature of the sputtering surface.
However, when an anodized aluminum plate is used as a substrate, the anodized layer is cooled to liquid nitrogen temperature on its one side which adheres to aluminum substrate and is heated on the other side by ion bombarding so the sputtered surface of the anodized layer is heated to a temperature above room temperature. The temperature difference between both the sides of anodized layer on aluminum substrate reaches to 100.degree. C. or more. Since the anodized layer has poor heat conductivity, this temperature difference results in cracks in the layer or peeling of the layer.
Further, the cooling of the substrate by liquid nitrogen causes high cost to manufacture a disk not only by precious liquid nitrogen consumption but also by requirement of complicated equipment and by impossibility of simultaneous sputtering of the two sides of the substrate.